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The compelling nature of the highly adapted functional surface structures found in biological systems accompanied by delicately tuned chemical processes, has inspired the design of materials with varied wetting properties and a vast range of applications. Identifying relations between surface structure, chemistry and wettability, is pivotal towards the mechanistic understanding of wetting phenomena. Here we demonstrate how electrochemically driven hydrogen adsorption/absorption induces, irreversibly, a superhydrophilic state in an amorphous CrFeNi-based multi-principal element alloy thin film with close to equimolar composition, i.e., in the class of medium/high entropy alloys. By employing films with sub-nanometer roughness to exclude the influence of geometry on wetting, we show that both the extent of wetting and its dynamics are governed by the rate of the underlying electrochemical reactions. The absorption of hydrogen into the matrix of the amorphous films as proved by thermal desorption spectroscopy, is proposed to partially protonate the electrochemically resilient Ta and Cr surface oxides through a hydrogen spillover phenomenon initiated by the adsorption of hydrogen on the electrochemically reduced Fe sites. Furthermore, atom probe tomography measurements reveal Cr segregation at the outermost surface layers of the film following cathodic treatment. The above processes strongly influence surface energetics resulting in the transition from a mildly hydrophilic state to an ultra-high substrate surface energy regime. Our work establishes a previously unknown physicochemical link between multi-principal element-alloys – hydrogen interactions and surface wettability, that is of significance in frontline research areas spanning electrochemical energy conversion/storage and catalysis to materials degradation and liquids actuation.

This study reveals the influence of crystal orientation on formation of growth twins in magnetron-sputtered coatings. A comparison between materials with low and high stacking fault energy (SFE) was made: CoCrFeMnNi (25 mJ/m(2)) and Ni (125 mJ/m(2)). The coatings were grown on a polycrystalline 316L stainless-steel substrate with near-random crystal texture, providing a comprehensive selection of samples on a single substrate. Electron backscatter diffraction was used to identify the film orientation, followed by transmission electron microscopy of selected regions. The presence and density of twins depended on both the material and the growth orientation. For Ni, nanotwins were observed on < 5 % of the substrate grains, on growth directions closest to (1 1 1) . All other film orientations grew epitaxially with a cube-on-cube relationship. For CoCrFeMnNi, nanotwinning was observed on 50 % of the substrate grains, deviating < 35 degrees from (1 1 1) . In the nanotwinned regions, the twin spacing was 10-100 nm for Ni and 2-20 nm for CoCrFeMnNi. The presence of nanotwins increased hardness in both materials. The mechanism behind these differences is discussed, together with other parameters for controlling twin density. Our results show that control of the growth process can be used for nanotwin-engineering in magnetron-sputtered materials with low SFE.

Ammonia is a promising energy vector and can be used as a hydrogen storage medium. Electrocatalytic ammonia syntheses using renewable energy are attractive low-temperature options to the Haber-Bosch high-temperature process, which releases CO₂ in the atmosphere and contributes to the greenhouse effect. High-entropy (HEA) alloys belong to a new class of materials that can provide single-phase stabilizations by mixing different species and are promising candidates to overcome scientific challenges posed by electrochemistry. Lithium-mediated ammonia synthesis is a way to get high-performance ammonia electrosynthesis from nitrogen at room temperature. In this work, we investigated lithium-mediated ammonia synthesis of a thin-film high-entropy catalyst of CoCrFeMnNi.

This thesis explores the design of metallic coatings for corrosion protection. The subject of the study was the new class of materials multicomponent alloys (MCAs, also known as high entropy alloys). They consist of near-equal concentrations of many (four or more) metals and are often reported to form a single phase with a simple crystal structure. Due to the complexity and range of possible MCA compositions, there is a need for design principles as guidelines for how the alloying elements can be chosen and combined. This work aimed at finding such principles through the systematic study of the synthesis and properties of three MCA systems. Their compositions were carefully chosen to answer fundamental questions about the materials class and the synthesis method and to generate conclusions that could be generalized to a larger group of MCAs. All three systems were based on the elements Cr, Fe, and Ni, and can therefore be considered an extension of stainless steels.

The first alloy was CoCrFeMnNi, which is well-known as a single-phase bulk MCA. A systematic exploration of the synthesis parameters showed that there are fundamental differences in the phase formation of CoCrFeMnNi through magnetron sputtering compared to typical bulk synthesis. Literature studies revealed that this conclusion can be generalized; single-phase MCAs should not necessarily be expected from magnetron sputtering. It was also shown that the choice of substrate and even the crystal orientation of the individual substrate grains strongly influenced the outcomes of synthesis, including the phase formation, growth rate, morphology, and the formation of stacking fault structures. 

Two novel alloy systems were also explored: CrFeNiTa and CrFeNiW. Ta and W were added to achieve an alloy with higher corrosion resistance than stainless steels and more generally, to examine the interplay between passivating elements in MCAs during corrosion. Based on geometrical considerations, it was predicted that equal amounts of Ta and W would be needed to protect alloys from corroding (less than 20 at%). It was found that the prediction was only valid for the CrFeNiTa alloy system. The reason behind this was explored and a new criterion was then proposed: In an MCA, each passivating element should have similar electrochemical nobility.

Further design possibilities were demonstrated by adding up to 50 at% carbon to the alloys. Thermodynamic calculations predicted decomposition into multiple metallic and carbide phases. However, the limited diffusion during magnetron sputtering suppressed the segregation. At lower carbon contents, the carbon-containing alloys were single-phase and amorphous. At higher carbon contents they formed alloy/amorphous carbon nanocomposites. The addition of carbon made the alloys stronger, more corrosion resistant, and more crack resistant. 

This study provides principles for designing new corrosion resistant high entropy alloys. The theoretical framework is a percolation model developed by Newman and Sieradzki that predicts the ability of an alloy to passivate, i.e., to form a protective surface oxide, based on its composition. Here, their model is applied to more complex materials than previously, namely amorphous CrFeNiTa and CrFeNiW alloys. Furthermore, the model describes a more complex passivation process: reforming the oxide layer above the transpassive potential of Cr. The model is used to predict the lowest concentration of Ta or W required to extend the passive region, yielding 11–14 at% Ta and 14–17 at% W. For CrFeNiTa, experiments reveal a threshold value of 13–15 at% Ta, which agrees with the prediction. For CrFeNiW, the experimentally determined threshold value is 37–45 at% W, far above the predicted value. Further investigations explore why the percolation model fails to describe the CrFeNiW system; key factors are the higher nobility and the pH sensitivity of W. These results demonstrate some limitations of the percolation model and offer complementary passivation criteria, while providing a design route for combining the properties of the 3d transition metal and refractory metal groups.

Magnetron sputter deposition of metal/carbon nanocomposites has been explored for many metals and a few alloys. In this paper, the formation of nanocomposites based on complex high entropy alloys (HEAs) was explored, focusing on the effect of the average carbon affinity on the phase formation. Two HEA systems were compared: CoCrFeMnNi and Cr26Fe27Ni27Ta20. For each alloy, around 20-50 at% carbon was added through combinatorial co-sputtering. Thermodynamic calculations predicted decomposition of these materials into multiple alloy phases, metal carbides, and, at higher concentrations, free graphitic carbon. Free carbon was found in the coatings at carbon concentrations above 28 and 33 at% for the CoCrFeMnNi and Cr26Fe27Ni27Ta20 systems, respectively, which agreed with the theoretical predictions. However, the segregation of metallic ele-ments and the formation of crystalline carbides were suppressed by the rapid quenching during deposition. All coatings were, instead, amorphous and consisted of either a single metallic phase or a mixture of a metallic phase and sp2- and sp3-hybridized carbon. Mechanical and electrochemical tests were performed, including in-situ fragmentation tests to quantify the crack resistance. The presence of free carbon made the coatings softer than the corresponding single-phase materials. Under tensile strain, the nanocomposite coatings formed a larger number of narrower cracks and exhibited less delamination at high strains.

This study explores the influence of a 316L stainless steel substrate on the magnetron sputtering of the Cantor alloy CoCrFeMnNi at different substrate bias. The study was carried out on a polycrystalline 316L substrate where the growth behavior of the coating could be investigated on grains with different orientations. By combining electron backscatter diffraction (EBSD) before and after deposition and characterization of the same area, it was possible to determine growth behaviour and surface morphologies on individual substrate grains. No strong influence of the substrate was observed at a floating bias. At a bias of -100V, however, the coating was strongly influenced by the orientation of the individual substrate grains.  Epitaxial coating grains with a smooth surface were observed on the [102]-oriented grains while a more columnar growth was observed on [111]-oriented grains.  Furthermore, a small difference in growth rate was observed on different substrate orientations. The growth behaviour could be related to differences in surface energies and diffusion rates on different surface orientations.

The corrosion resistances of near equimolar CoCrFeMnNi magnetron-sputtered thin films with different carbon concentrations were examined in 0.05 M HCl and 0.05 M H2SO4. Polarization curves were recorded with different scan rates with and without reducing the native oxide. The results showed that the carbon concentration and the experimental conditions affected the electrochemical behaviour mainly in the Cr transpassive region. At potentials above 850 mV, the carbon-containing films were more corrosion resistant in 0.05 M HCl than in 0.05 M H2SO4 due to a lower carbon oxidation rate in 0.05 M HCl, facilitating the formation of a Mn-rich oxide layer. (C)& nbsp;2021 The Author(s). Published by Elsevier Ltd.& nbsp;

We demonstrate a combinatorial approach integrating ion implantation followed by thermal annealing and simultaneous in situ ion beam analysis with thermal desorption spectroscopy in a single set-up. Atomic and molecular deuterium ions of 3 keV were implanted into bulk tungsten with doses exceeding 1 x 10(22) ions m(-2). Depth profiling of both, protium and deuterium was performed by elastic recoil detection analysis, while simultaneously the outgassing rates of molecular deuterium by thermal desorption spectroscopy were monitored during temperature ramps from room temperature to approximate to 1400 K. The combination of the two techniques in situ is shown capable to identify the distinct retention behavior of deuterium at different temperatures and in different reservoirs, e.g. located close to the surface and diffused deep into the material. Ex-situ scanning electron microscopy confirmed blister formation, and recovery of the initial surface morphology after high temperature annealing, in analogy to comprehensive ex-situ studies.

This study explores carbon addition as a materials design approach for simultaneously improving the hardness, crack resistance, and corrosion resistance of high entropy thin films. CoCrFeMnNi was selected as a starting point, due to its high concentration of weak carbide formers. The suppression of carbides is crucial to the approach, as carbide formation can decrease both ductility and corrosion resistance. Films with 0, 6, and 11 at.% C were deposited by magnetron co-sputtering, using a graphite target and a sintered compound target. The samples with 0 at.% C crystallized with a mixture of a cubic closed packed (ccp) phase and the intermetallic χ-phase. With 6 and 11 at.% C, the films were amorphous and homogenous down to the nm-scale. The hardness of the films increased from 8 GPa in the carbon-free film to 16 GPa in the film with 11 at.% C. Furthermore, the carbon significantly improved the crack resistance as shown in fragmentation tests, where the crack density was strongly reduced. The changes in mechanical properties were primarily attributed to the shift from crystalline to amorphous. Lastly, the carbon improved the corrosion resistance by a progressive lowering of the corrosion current and the passive current with increasing carbon concentration.